It is known that NOx formation increases with increasing flame temperature and with increasing residence time in the combustor. It is therefore theoretically possible to reduce NOx emissions from a combustor by reducing flame temperature and/or the time at which the reacting gases remain at peak temperatures. In practice, however, this is difficult to achieve because of the turbulent diffusion flame characteristics of present day gas turbine combustors. In such combustors, combustion takes place in a thin layer surrounding the evaporating liquid fuel droplets at a fuel/air equivalence ratio near unity, regardless of the overall reaction zone equivalence ratio. Since this is the condition which results in the highest flame temperature, relatively large amounts of NOx are produced. As a result, the conventional single stage, single fuel nozzle spray atomized combustors may not meet newly established emission standards regardless of how lean the nominal reaction zone equivalence ratio is maintained.
It is also known that significant reductions in NOx emissions can be achieved by injection of water or steam into the combustor reaction zone. However, such injection has many disadvantages including an increase in the system complexity and high water treatment costs.
The problem of realizing low NOx emissions develops further complexity where it is necessary to meet other combustion design criteria. Among such criteria are those of good ignition qualities, good cross firing capability, stability over the entire load range, large turn-down ratio, low traverse number, long life and ability to operate safely and reliably.
In commonly owned U.S. Pat. No. 4,292,801, there is described a dual stage-dual mode low NOx combustor for combustion turbine application. This combustor includes a throat section which separates the primary and secondary stages. Specifically, the combustor described in the above mentioned patent uses a throat section with film cooling air introduced via cooling slots formed by rolled ring sheet metal. This is a standard method of wall cooling used on current production combustors in gas turbine service. This method of cooling introduces a relatively large mass flow of cooling air at compressor discharge air temperature along the surface of the combustor throat section facing the combustion reaction zone. This method of cooling results in a relatively low temperature boundary layer with a very lean fuel/air mixture. It is known, however, that chemical reactions are quenched in this lean, low temperature boundary layer with the result that the emissions of carbon monoxide (CO) and unburned hydrocarbons (UHC) at the combustor exit are increased.
An alternative cooling scheme for the dual stage-dual mode low NOx combustor throat which has been applied is the use of vigorous backside convection cooling obtained by impinging cooling air jets. This method does not have the disadvantage of film air cooling which quenches chemical reactions. However, the backside cooling method is limited to clean natural gas fuel at current production gas turbine cycle conditions, because heat rejection from the throat section liner walls is not adequate for liquid fuels and advanced machine cycle conditions using only backside cooling methods. This limited heat rejection capability results in throat section liner wall temperatures which are too high for long term durability in gas turbine service when the dual stage-dual mode low NOx combustor is operated on liquid fuels and/or advanced machine cycle conditions.
The object of this invention is to provide a transpiration cooled throat section in a dual stage-dual mode low NOx combustor of the type described in U.S. Pat. No. 4,292,801 with sufficient durability for gas turbine service at current production cycle conditions using liquid fuels and at advanced machine cycle conditions using a variety of gaseous and liquid fuels. It is also an object of this invention to provide a method of cooling the throat section of the dual stage-dual mode low NOx combustor which does not quench chemical reaction in the combustor reaction zone, and which does not result in increased carbon monoxide and unburned hydrocarbon emissions at the combustor exit. It is further an object of this invention to prevent the formation of deposits on the surface of the throat section of the dual stage-dual mode low NOx combustor when operating on liquid fuel in the premixed mode.
In transpiration cooling, air or other fluid effuses through a porous structure into the boundary layer on the hot gas side in order to maintain the internal structure at a temperature below that of the hot gas stream. Thus, cooling is accomplished both by the absorption of heat within the wall by the coolant, as well as by the alteration of the boundary layer to thereby reduce the skin friction and heat transfer through the boundary layer.
Transpiration cooling in gas turbines is not new, reference being made to U.S. Pat. Nos. 3,557,553; 4,004,056; 4,158,949; 4,180,972; 4,195,475; 4,232,527; 4,269,032; 4,302,940; and 4,422,300.
Nevertheless, transpiration cooling has not heretofore been utilized in the throat region of a dual stage-dual mode combustor of the type utilized in this invention.
In an exemplary embodiment of the invention, the throat region is formed by converging and diverging wall sections, relative to the direction of fuel/air flow. The throat region thus presents a reduced diameter portion relative to the first and second combustion chambers.
In this exemplary embodiment, an outer liner surrounds the throat region to provide a cooling air plenum. A plurality of air metering holes are provided in the liner to provide the required cooling air (from the compressor) mass flow and pressure within the plenum.
The converging and diverging wall sections of the throat region are constructed of a porous metal material which permits transpiration cooling air injection into the throat region. It will be understood that the size and number of air metering holes and the porosity of the throat wall sections are chosen to provide that amount of transpiration cooling air necessary to match the local heat load which varies over the inner surfaces of the throat wall sections.
Accordingly, the present invention provides a method and apparatus for achieving a significant reduction in NOx emissions from a gas turbine without aggravating ignition, unburned hydrocarbon or carbon monoxide emission problems. More particularly, the dual stage-dual mode low NOx combustor of this invention includes first and second combustion chambers or stages interconnected by a throat region. Fuel and mixing air are introduced into the first combustion chamber for premixing. The first chamber includes a plurality of fuel nozzles positioned in circumferential orientation about the axis of the combustor and protruding into the first stage through the rear wall of the first chamber. Additional fuel is introduced near the downstream end of the first combustion chamber, and additional air is added in the throat region for combustion in the second combustion chamber.
The combustor is operated by first introducing fuel and air into the first chamber for burning therein. Thereafter, the flow of fuel is shifted into the second chamber until burning in the first chamber terminates, followed by a reshifting of fuel distribution into the first chamber for mixing purposes with burning occurring in the second chamber. The combustion in the second chamber is rapidly quenched by the introduction of substantial amounts of dilution air into the downstream end of the second chamber to reduce the residence time of the products of combustion at NOx producing temperatures, thereby providing a motive force for the turbine section which is characterized by low amounts of NOx, carbon monoxide and unburned hydrocarbon emissions.
At the same time, as the cooling air passes through the throat section walls, heat is transferred to the cooling air with the result that the cooling air is injected into the combustion reaction zone at temperatures close to the inside surface temperature of the throat section walls. Because less air is used in transpiration cooling vis-a-vis film air cooling, and because air enters the reaction zone at higher temperatures with transpiration cooling than with film air cooling, the cooling air in accordance with this invention will not result in quenching of chemical reactions in the boundary layer.
Moreover, in the premixed operating mode when using liquid fuels, droplets of liquid fuel will exist at the exit of the primary stage. These droplets can impinge on the inner surfaces of the throat section walls and result in the formation of deposits when the backside cooling technique is used. The present invention prevents the formation of such deposits because the transpiration cooling air removes any droplets on the inner surfaces of the throat wall sections.
In accordance with the broader aspects of one exemplary embodiment of the invention, therefore, there is provided a low NOx combustor for a gas turbine comprising first and second combustion chambers interconnected by a throat region, the throat region including converging and diverging wall sections constructed of porous metal material; and a cooling air plenum surrounding the throat region and including at least one air metering opening communicating the plenum with a cooling air source.
In accordance with another broad aspect of the invention, a method of cooling a reduced diameter throat region in a dual stage-dual mode gas turbine combustor is provided which includes the steps of:
a) providing a plenum surrounding the reduced diameter throat region;
b) supplying metered amounts of cooling air to said plenum;
c) providing porous metal wall sections to define the reduced diameter throat region; and
d) introducing transpiration cooling air from the plenum through the porous metal wall sections to the throat region.
Other objects and advantages of the present invention will become apparent from the detailed description which follows.